1. Field of the Invention
The invention relates to the field of dynamic magnetic information storage or retrieval. More particularly, the invention relates to the field of automatic control of a recorder mechanism. In still greater particularity, the invention relates to track centering using a servo pattern. By way of further characterization, but not by way of limitation thereto, the invention is a method for detecting a frame edge by generating a digital signal in response to a change in signal frequency.
2. Description of the Related Art
Magnetic tape recording has been utilized for many years to record voice and data information. For information storage and retrieval, magnetic tape has proven especially reliable, cost efficient and easy to use. In an effort to make magnetic tape even more useful and cost effective, there have been attempts to store more information per given width and length of tape. This has generally been accomplished by including more data tracks on a given width of tape. While allowing more data to be stored, this increase in the number of data tracks results in those tracks being more densely packed onto the tape. As the data tracks are more closely spaced, precise positioning of the tape with respect to the tape head becomes more critical as errors may be more easily introduced into the reading or writing of data. The tape-tape head positioning may be affected by variations in the tape or tape head, tape movement caused by air flow, temperature, humidity, tape shrinkage, and other factors, especially at the outside edges of the tape.
In order to increase data track accuracy, servo stripes have been employed to provide a reference point to maintain correct positioning of the tape with respect to the tape head. One or more servo stripes may be used depending upon the number of data tracks which are placed upon the tape. The sensed signal from the servo stripe is fed to a control system which moves the head and keeps the servo signal at nominal magnitude. The nominal signal occurs when the servo read gap is located in a certain position relative to the servo stripe. Referring to FIG. 1, a one-half inch wide length of magnetic tape 11 may contain up to 288 or more data tracks on multiple data bands 12. With such a large number of data tracks it may be desirable to include up to five or more servo bands 13 to improve data read and write function performance. Servo bands 13 may utilize various patterns or frequency regions to allow precise tape to tape head positioning thus allowing a data read head to more accurately read data from data bands 12.
Referring to FIG. 2, a portion of a conventional servo stripe 13 is shown having two frames 14 and 15. A first frequency signal 16 is written across the width of servo stripe 13. An erase frequency is written over first frequency signal 16 in a predetermined pattern such as five rectangles 17 in each of frames 14 and 15. The five rectangles 17 in each frame result in nine horizontal interfaces 18 between frequency signal 16 and erase patterns 17 as the tenth edge 19 along the bottom is ignored. A dashed line 21 illustrates the alignment of a read gap 22 in a tape read head 23.
Referring to FIG. 2, if the alignment of read gap 22 with servo pattern 13 is as shown, dotted line 21 passes along one of edges 18 and through the center of gap 22. If the servo pattern on the tape is passed right to left over gap 22, then read gap 22 will alternate between reading frequency 16 across the full width 24 of gap 22 in areas 25 and frequency 16 across one half of read gap 22 and an erase frequency from patterns 17 across the other half of width 24 in areas 26.
FIG. 3 shows the read frequency signals from one frame 14 or 15 as read by head gap 22 in FIG. 2. The amplitude of the signal is larger in areas 25 where frequency area 16 passes over the full width 24 of head gap 22. The amplitude of the signal is about half as large in area 26 when one half of width 24 reads frequency area 16 and the other half reads erase patterns 17. The servo control system in a tape drive uses the ratio of the full signal amplitude in field 25 to the half signal amplitude in field 26 to stay on track. For the on track position shown by dotted line 21 in FIG. 2 the ratio will be exactly one-half because one half of read gap width 24 is over area 16 and one-half is over erase pattern 17.
Referring to FIG. 1 and FIG. 2, if tape 11 and hence servo stripe 13 move down with respect to tape head 23, then, in field 26, more of area 16 and less of pattern 17 will be over head gap 22. Referring to FIG. 3, if more of area 16 is read, then the signal in field 26 will increase and this will be sensed by the controller. Conversely, if tape 11 and thus servo stripe 13 move up in FIGS. 1 and 2, then head gap 22 will see less of area 16 and more of pattern 17 across width 24. Thus, the signal in area 26 of FIG. 3 will decrease in amplitude proportionately to this movement. In this way the tape controller can sense the position of the tape 11 with respect to the read gap 22 and move head 23 to keep the head gap 22 aligned with the servo stripe along line 21. This alignment ensures precise reading of a data track in data bands 12 by the data read head (not shown). While this system can result in more precise positioning of the tape head 23 with respect to tape 11, a difficulty can arise in that the controller must be able to determine in which field, 25 or 26, it is in at the time the signal is read. That is, there must be synchronization between the time the signal in field 25 is sampled and the time the signal in field 26 is sampled.
Referring again to FIG. 3, the change in signal amplitude in moving to or from field 25 to or from field 26 could be used to determine in which field/area on the servo stripe read gap 22 is located. That is, if the signal drops to about one-half amplitude, it can be assumed that gap 22 is sensing movement of the servo pattern on the tape from field 25 into field 26. Conversely, if the signal amplitude approximately doubles, it can be assumed that gap 22 is sensing tape movement to field 25 from field 26. However, this method is prone to error for a number of reasons. If head gap 22 is aligned such that it passes between erase patterns 17, only signal 16 will be read and there will be no amplitude change in the signal from areas 25 to areas 26 or vice versa. In effect the control system which positions the head with respect to the tape is lost. The system controller does not know whether gap 22 is in region 25 or 26. Another possible source of error is noise. Because the signals are analog and contain a significant amount of noise, it may be difficult to determine the change in amplitude as gap 22 senses movement from field 25 to field 26 and vice versa. It would be desirable to have a system in which the servo control circuitry could reliably determine in which region read gap 22 is then located.